In a drawing process of quartz glass for manufacturing an optical fiber, a coating resin is immediately applied to the circumference of an optical fiber to prevent reduction in strength of the optical fiber. For the identification, a colored layer is provided. A UV curing resin is mainly used as the coating resin for an optical fiber. A urethane acrylate or epoxy acrylate is used as the UV curing resin.
In an optical fiber, transmission losses are increased by various external stresses and microbending caused thereby. For this reason, to protect the optical fiber from such external stresses, an optical fiber strand is generally provided with a coating having a two-layer structure including a soft layer and a hard layer. A soft resin having a relatively low Young's modulus is used for the inner layer, which is into contact with quartz glass, as a buffer layer (hereinafter, primary layer), while a hard resin having a relatively high Young's modulus is used for the outer layer as a protective layer (hereinafter, secondary layer). Generally, a resin having a Young's modulus of 3 MPa or less is used for the primary layer, and a resin having a Young's modulus of 500 MPa or more is used for the secondary layer.
In a method of manufacturing an optical fiber, a preform mainly containing quartz glass is melted by heating in a drawing furnace, and a quartz glass optical fiber is drawn from the preform. Then, a liquid UV curing resin is applied to the quartz glass optical fiber using a coating die. Subsequently, the resultant is irradiated with UV rays to cure the UV curing resin. By such a method, the optical fiber is coated with the primary layer and the secondary layer, and thereby an optical fiber strand is manufactured. Furthermore, in the subsequent step, the circumference of the optical fiber strand thus obtained is coated with a coating layer made of a colored resin. Hence, a colored coated optical fiber is manufactured.
In this description, such a glass optical fiber coated with a primary layer and a secondary layer is referred to as an optical fiber strand; an optical fiber strand whose circumference is further coated with a coating layer made of a colored resin is referred to as a colored coated optical fiber; furthermore, multiple colored coated optical fibers arranged on a plane and collectively coated with a ribbon resin are referred to as an optical fiber ribbon.
Meanwhile, in order to manufacture a highly reliable colored coated optical fiber which prevents an increase in transmission loss even when used under an immersed condition in 60° C. hot water, Patent Literature 1 proposes to increase the Young's modulus of a colored layer higher than the Young's modulus of a secondary layer, and to increase the glass transition temperature of the colored layer higher than the glass transition temperature of the secondary layer.